Quantifying HIV-1 Exposure to Illuminate Resistance to Infection

In the early 1990s, the research group of Frank Plummer at
the University of Manitoba drew considerable attention -- and some controversy -- when they reported that, among a large cohort of female sex workers in Nairobi, a subset showed evidence of resistance to HIV infection. The evidence emerged over the course of a long-term study that found that women starting sex work faced a very high risk of seroconverting in the first two years. However, for a subset of women who remained HIV-negative, the risk of becoming infected
subsequently declined significantly over time, which Plummer and colleagues
interpreted as evidence of resistance to acquisition. Despite initial
skepticism, the findings prompted efforts to identify individuals with possible
resistance to HIV in other settings, such as among serodiscordant couples (in
which one partner is HIV-positive and the other HIV-negative). There is now a
substantial amount of literature on the topic, as well as ongoing workshops and
research collaborations, but one of the challenges in the field is that there
are no widely accepted criteria for defining high exposure to HIV (an important
part of assessing whether an individual may be resistant as opposed to simply
unexposed).

In a new paper in the Journal of Infectious Diseases, Romel
Mackelprang and colleagues from the Partners in Prevention HSV/HIV Transmission
Study describe a method for quantifying HIV exposure among serodiscordant
couples, with the goal of enhancing efforts to identify individuals with
potential resistance to infection. The study draws on data from a trial
involving 3,408 serodiscordant heterosexual African couples. Risk factors that
were associated with HIV transmission -- such as unprotected sex, viral load in
the HIV-positive partner, and genital ulcer disease -- were used to create
"exposure scores" for the study participants. Unsurprisingly, high exposure was
associated with a 6.9-fold increased risk of infection compared to low
exposure. But these scores also identified a subset of 475 individuals with
persistently high exposure who remained seronegative. Echoing Frank Plummer's
findings, the risk of HIV infection appeared to decline over the time in the
high exposure group, while remaining constant among participants with lower
exposure.

To further validate these findings, the researchers applied
a simplified version of the exposure-score model to an independent cohort of
485 serodiscordant couples from Kampala, Uganda, and Soweto, South Africa.
Again, the analysis identified a group (comprising 48 individuals) with high
exposure but a decreasing risk of acquiring HIV infection over time. The study
authors conclude: "our approach to estimating HIV-1 exposure using longitudinal
data from both partners in HIV-1serodiscordant couples provides an objective
tool to identify subsets of HESN [HIV-1exposed seronegative] individuals to
target for identification of host factors protecting against HIV-1." Additional
details on the exposure scoring system are provided as supplementary data on the journal website.

This article was provided by Treatment Action Group. It is a part of the publication Michael Palm HIV Basic Science, Vaccines & Cure Project.

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